The present invention relates to a cathode of a cathode-ray tube used for a display such as a television receiver or computer monitor, particularly to a method for producing an oxide cathode including a specific emissive material layer.
FIG. 8 illustrates an oxide cathode in which a porous emissive material layer 9 is formed on a cathode substrate 3 on one end of a sleeve 2 containing a heater coil 1, which is known widely as a cathode of a cathode-ray tube. JP 5(1993)-74324A discloses one conventional example of such an oxide cathode, in which an emissive material layer is separated into an upper layer (surface side) and a lower layer (substrate side), and the particle size of the emissive material in the upper layer is made smaller than that of the emissive material in the lower layer. Accordingly, the surface roughness of the emissive material layer can be decreased to improve flatness, so that the angle of thermionic emission (emittance) can be decreased, and distortion of the current density distribution of emission electrons can be eliminated. Thus, a cathode-ray tube with excellent resolution can be realized.
However, in this case, as the particle size of the upper layer of the emissive material layer is smaller than that of the lower layer, because the particles forming the upper layer are fine, its bulk density is increased, and its porous structure is lost easily. Thus, the electron emission characteristic of the cathode is reduced easily when the cathode-ray tube is operated for a long time.
It is an object of the present invention to provide a method for producing an oxide cathode including a specific emissive material layer with high resolution, without deterioration of the electron emission characteristic of the cathode when operated for a long time.
To solve the above problem, the present invention provides a method for producing an oxide cathode including a sleeve containing a heater coil, a cathode substrate provided on one end of the sleeve, and an emissive material layer formed by thermally decomposing an alkaline earth metal carbonate layer adhered onto the cathode substrate, which method includes: adhering the alkaline earth metal carbonate onto the cathode substrate so that the alkaline earth metal carbonate has a bulk density of at least 0.5 g/cm3 but not more than 0.8 g/cm3; then pressing the alkaline earth metal carbonate so that the bulk density becomes not more than 0.9 g/cm3, thereby forming the carbonate layer; and then thermally decomposing the carbonate layer in vacuum.
According to the method of the present invention, the flatness of the surface of the emissive material layer can be improved without damaging its porous structure.
In the method of the present invention, it is preferable that the pressure of the pressing is at least 1.5xc3x97105 Pa but not more than 3.5xc3x97105 Pa. Accordingly, the bulk density and the surface roughness of the emissive material layer can be optimized.
In the method of the present invention, it is preferable that the thickness of the carbonate layer after the pressing is at least 40 xcexcm but not more than 90 xcexcm. Accordingly, a decrease in the emission current of the oxide electrode can be inhibited, while the emissive material layer can be prevented from peeling.
In the method of the present invention, it is preferable that the surface roughness of the carbonate layer after the pressing is not more than 13 xcexcm. Accordingly, distortion of the current density of emission electrons can be eliminated.
Furthermore, it is preferable that the alkaline earth metal carbonate has an average particle size of at least 2 xcexcm and a maximum particle size of not more than 13 xcexcm. Accordingly, the porosity of the emissive material can be maintained.
In the method of the present invention, it is preferable that the bulk density of the carbonate layer after the pressing is at least 0.6 g/cm3 but not more than 0.9 g/cm3.
Furthermore, it is preferable that the thermal decomposition is carried out at a temperature of 900 to 1000xc2x0 C.
Furthermore, it is preferable that the thermal decomposition is carried out at a pressure of 1xc3x9710xe2x88x926 to 1xc3x9710xe2x88x922 Pa.
Accordingly, a cathode-ray tube with excellent resolution in which moire is invisible or hardly visible can be realized.
In the present invention, it is preferable that the ratio of remaining emission current after being operated for 2000 hours at a temperature of the emissive material layer of 850xc2x0 C. with an emission current density of 2 A/cm2 is at least 80%, when considering the initial value as 100%.
In the present invention, it is preferable that the alkaline earth metal carbonate is a binary carbonate of barium and strontium or a ternary carbonate of barium, strontium and calcium.